Electrical shocks are harmful to a heart attack victim, although they are necessary for survival if the victim's heart is in a state of ventricular fibrillation (VF). Electrical shocks that are delivered to a heart that has a weak but detectable beat (which produces QRS signals) harm the patient by interfering with the capability to pump blood and by damaging the heart muscle. U.S. Pat. No. 6,393,316 describes a method for determining whether or not to apply a defibrillating shock.
If other people are present at the onset of a heart attack and especially when loss of consciousness occurs, bystander CPR (cardiopulmonary resuscitation) greatly increase the chance of survival. Basic to CPR is the initiation and continuation of chest compressions, and sometimes also the interval delivery of breaths into the victim's mouth or nose. ECG (electrocardiograhpic) signals generated by electrodes attached to the victim's chest, can be used to determine whether or not to apply a defibrillation shock to the patient, to reverse ventricular fibrillation. Currently, this requires the rescuer to stop chest compression and to “stand back” away from the patient for many seconds to preclude shocking the rescuer and to stop adding chest compression artifacts to the ECG signals so a “clean” ECG signal can be analyzed. This interruption of chest compressions adversely affects the survival possibility of the victim.
Chest compressions produce additional signal components that represent artifacts on the victim's ECG signals that are transmitted through electrodes applied to the patient's chest. Such artifacts due to chest compressions are typically of much greater amplitude than the actual ECG signals of the victim when the chest is not compressed. As a result, portions of an ECG signal arising from the heart are masked by the artifacts. This could preclude accurate analysis of the patient's condition and a decision to apply a defibrillation shock.
Conventional automatic external defibrillators, which automatically analyze the ECG and generate defibrillation shocks, avoid this problem by instructing the rescuer to leave the victim untouched for as long as perhaps 25 seconds so the ECG signals from the victim are not corrupted to facilitate their analysis. The problem with this approach is that the suspension of chest compressions reduces circulation of blood in the victim's body and therefore compromises survival. An alternative is to shorten the interval of “step back, don't touch the patient”. A protective sheet, described in U.S. Pat. No. 6,360,125, can be placed over the victim's chest to avoid transmitting a shock to the rescuer so as to reduce the “step back, don't touch” interval to slightly more than required to obtain “quiet” ECG signals to analyze. However, regardless of the length of the “quiet” period during which the ECG is evaluated to determine if the patient's heart is experiencing VF, the chest compressions which maintain circulation are interrupted. Prior studies have demonstrated that the cessation of compressions by as little as 10 seconds reduces the probability of successful defibrillation by as much as 50%.